3,014 research outputs found
A Berger type normal holonomy theorem for complex submanifolds
We prove a kind of Berger-Simons' Theorem for the normal holonomy group of a complex submanifold of the projective spac
A geometric proof of the Karpelevich-Mostow's theorem
In this paper we give a geometric proof of the Karpelevich's theorem that
asserts that a semisimple Lie subgroup of isometries, of a symmetric space of
non compact type, has a totally geodesic orbit. In fact, this is equivalent to
a well-known result of Mostow about existence of compatible Cartan
decompositions
Corrigendum for "A geometric proof of the Karpelevich-Mostow theorem"
Corollary 2.3 in our paper "A geometric proof of the Karpelevich-Mostow
theorem", Bull. Lond. Math. Soc. 41 (2009), no. 4, 634-638, is false. Here we
give a counterexample and show how to avoid the use of this corollary to give a
simpler proof of Karpelevich-Mostow theorem. We also include a short discussion
of the original proof by Karpelevich
On the stability of Hamiltonian relative equilibria with non-trivial isotropy
We consider Hamiltonian systems with symmetry, and relative equilibria with
isotropy subgroup of positive dimension. The stability of such relative
equilibria has been studied by Ortega and Ratiu and by Lerman and Singer. In
both papers the authors give sufficient conditions for stability which require
first determining a splitting of a subspace of the Lie algebra of the symmetry
group, with different splittings giving different criteria. In this note we
remove this splitting construction and so provide a more general and more
easily computed criterion for stability. The result is also extended to apply
to systems whose momentum map is not coadjoint equivariant
A Cellular Automaton Model for the Traffic Flow in Bogota
In this work we propose a car cellular automaton model that reproduces the
experimental behavior of traffic flows in Bogot\'a. Our model includes three
elements: hysteresis between the acceleration and brake gaps, a delay time in
the acceleration, and an instantaneous brake. The parameters of our model were
obtained from direct measurements inside a car on motorways in Bogot\'a. Next,
we simulated with this model the flux-density fundamental diagram for a
single-lane traffic road and compared it with experimental data. Our
simulations are in very good agreement with the experimental measurements, not
just in the shape of the fundamental diagram, but also in the numerical values
for both the road capacity and the density of maximal flux. Our model
reproduces, too, the qualitative behavior of shock waves. In addition, our work
identifies the periodic boundary conditions as the source of false peaks in the
fundamental diagram, when short roads are simulated, that have been also found
in previous works. The phase transition between free and congested traffic is
also investigated by computing both the relaxation time and the order
parameter. Our work shows how different the traffic behavior from one city to
another can be, and how important is to determine the model parameters for each
city.Comment: 14 pages and 13 figures (gzipped tar file). Submitted to
Int.J.Mod.Phys.C. Minor changes, specially at references and typoes, plus a
clearer summary of the CA rule
Hijas (< 20 aƱos, con actividad sexual) y madres. Diferencias en edades de la menarquia y del primer coito. Estudio prospectivo de 1 aƱo
Objetivo: Comparar las edades a la menarquia y del primer coito en mujeres < 20 aƱos con actividad sexual, presentadas a la consulta como hijas solas o hijas con sus madres. DiseƱo: Estudio prospectivo de 1 aƱo. Se establecieron dos grupos de pacientes: hijas solas, 40 casos, e hijas con madres (H+M), grupo pareado, 46 casos. A travĆ©s de la entrevista personal se adquirieron los siguientes datos: edad, edad a la menarquia, fĆ³rmula menstrual, antecedentes mĆ©dicos y quirĆŗrgicos, IMC (Ćndice de masa corporal), edad al primer coito, contracepciĆ³n, vacunaciĆ³n de virus del papiloma (VPH: sĆ, no), motivo de consulta, actividad (estudia o trabaja). Y en la madre, ademĆ”s, paridad y edad a la menopausia (si procede). Resultados: Globalmente, las hijas son de 15-19 aƱos, media 17 aƱos. Edad a la menarquia, 9-16 aƱos, media 12 aƱos. Edad al primer coito, 11-19 aƱos, media 15 aƱos. IMC, entre 15 y 32, media 21. Presentaban oligomenorreas en 50%. En 20 % no usaban contracepciĆ³n.
Las madres, de 32-63 aƱos, media 47 aƱos. Paridad media 2,36 (0-5). Edad a la menarquia, media 12 aƱos. Edad al primer coito, media 18 aƱos.
No hubo diferencias en la edad a la menarquia, pero sĆ hubo en la edad al primer coito: entre las hijas globalmente, 15,45 aƱos, versus 18,30 aƱos en las madres (p< 0,001), y entre las hijas con madres, H+M, y sus madres, 16,17 aƱos versus 18,30 aƱos (p< 0,001).
El coito fue anterior en las H+M (32 casos, 69,56%) que en sus madres. La diferencia entre edad de la menarquia y edad al primer coito en las H+M fue 3,58 Ā±2,16 aƱos, y en las madres 5,91Ā±3,92 aƱos.
Conclusiones:
1. Hay diferencias en la edad al primer coito, entre hijas y sus madres, p< 0,001: mƔs de 2 aƱos antes, 16,17 versus 18,30 aƱos.
2. La diferencia entre la edad de la menarquia y la edad al primer coito fue 3,18 aƱos en las hijas globalmente, y 5,91 aƱos en las madres (p< 0,001).
3. Las hijas que tuvieron el primer coito a edad normal (16-18 aƱos) comenzaron estudios universitarios, p< 0,05, mƔs que las que inician el coito a edad mƔs precoz
- ā¦